This invention is directed to a hydraulic system and more particularly a closed circuit hydraulic system that utilizes a bypass valve to maintain minimum low side pressure.
In a closed circuit, a pump and a motor are used to transfer power from a power source connected to a pump shaft to a load connected to the motor shaft. In a closed circuit system, hydraulic fluid flows from the pump to the motor and then from the motor directly back to the pump to form a “loop”.
To transfer power from the pump to the motor, hydraulic fluid pressure has to be higher on one side of the loop as compared to the other. The higher of the two pressures is called the high side and the lower of the two pressures is called the low side. A charge pump is often used to make up for leakages in the system to maintain pressure on the low side. Sometimes a closed circuit pump is connected directly to one or more hydraulic cylinder and hydraulic fluid is returned directly from the hydraulic cylinders back to the pump.
Often, in a closed circuit system a minimum low side pressure needs to be maintained while the system is operating. Maintaining a minimum low side pressure becomes problematic when a shock load occurs, resulting in a steep pressure rise rate. When this occurs, fluid is compressed causing the hose or conduit to expand, and oil is pumped into the high side. Meanwhile, significantly less fluid is returned from the motor back to the pump, which causes the pressure on the low side to lower due to the charge pump's inability to compensate for the fluid required during the shock load. This event is commonly referred to as a low loop event which can cause damage to the pump and the motor.
To address fluctuations in the low side, accumulators and large charge pumps have been used. Accumulators are a pressure reservoir that contains uncompressed fluid under pressure. Although useful, accumulators are expensive to incorporate into a system and costly to maintain due to the level of skill a serviceperson must have. Accumulators are unreliable due to issues related to running out of fluid, which renders them ineffective.
Alternatively or in addition, larger charge pumps or extra-large charge pumps are added to a system. These types of charge pumps are less efficient due to the heat added to the system.
Thus, it is a primary objective of this invention to improve upon the state of the art.
Another objective of this invention is to prevent low loop events during a shock load.
Yet another objective is to eliminate the need for accumulators and other low efficiency, high cost, or high maintenance elements used to avoid low loop events.
These and other objectives, features, and advantages of the invention will become apparent from the specification and claims.